vagrant: use the openbsd 7.1 box

openbsd 6.9 was removed from the ftp servers and
the vagrant box stopped working. so we now use the
same box as in master branch.

Vagrantfile

@@ -68,9 +68,11 @@ def packages_openbsd
   return <<-EOF
     pkg_add bash
     chsh -s bash vagrant
+    pkg_add xxhash
     pkg_add lz4
     pkg_add zstd
     pkg_add git  # no fakeroot
+    pkg_add openssl%1.1
     pkg_add py3-pip
     pkg_add py3-virtualenv
   EOF
@@ -355,7 +357,7 @@ Vagrant.configure(2) do |config|
   end
 
   config.vm.define "openbsd64" do |b|
-    b.vm.box = "generic/openbsd6"
+    b.vm.box = "openbsd71-64"
     b.vm.provider :virtualbox do |v|
       v.memory = 1024 + $wmem
     end